I've used Sigil in the past to modify other ebooks with great success. I decided it was time to create my own ebook and have spent the last few days scanning a book I own and using Sigil to create the ebook.

It looks great in Sigil and it looks great in calibre. In ADE and on my PRS-505, however, the css doesn't seem to be applied to the text.

I've got the link to the css file in the <head></head> section of each chapter

Actually, just as an experiment, I pasted my css code above directly into one of the chapter headers, between <style></style> headers, and that chapter appeared the way I expected - so I must have not done something correctly linking the style sheet.